linux-kernel - Re: [PATCH 3/5] rust: alloc: add Vec::push_within

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Message-ID: <CAJ-ks9kFU+epCF3C4yUo5E2dVeyeBf+Fuc+rWe+2CM1OQvRDTA@mail.gmail.com>
Date: Fri, 21 Mar 2025 11:22:20 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] rust: alloc: add Vec::push_within_capacity

On Thu, Mar 20, 2025 at 9:57 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> This introduces a new method called `push_within_capacity` for appending
> to a vector without attempting to allocate if the capacity is full. Rust
> Binder will use this in various places to safely push to a vector while
> holding a spinlock.
>
> The existing Vec::push method is reimplemented in terms of the new
> method.
>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/alloc/kvec.rs | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 9943358c70aa63f5ad7ed9782cb8879d7a80a8fb..df930ff0d0b85b8b03c9b7932a2b31dfb62612ed 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -284,6 +284,31 @@ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
>      /// ```
>      pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
>          self.reserve(1, flags)?;
> +        let err = self.push_within_capacity(v);
> +        // SAFETY: The call to `reserve` was successful, so `push_within_capacity` cannot fail.
> +        unsafe { err.unwrap_unchecked() };

I'd prefer an unsafe inner helper to this `unwrap_unchecked` call --
this safety comment is actually describing the semantics of a safe
function which can change without necessarily triggering scrutiny of
its callers.

> +        Ok(())
> +    }
> +
> +    /// Appends an element to the back of the [`Vec`] instance.
> +    ///
> +    /// Fails if the vector does not have capacity for the new element.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let mut v = KVec::with_capacity(10, GFP_KERNEL);
> +    /// for i in 0..10 {
> +    ///     v.push_within_capacity(i).unwrap();
> +    /// }
> +    ///
> +    /// assert!(v.push_within_capacity(11).is_err());
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    pub fn push_within_capacity(&mut self, v: T) -> Result<(), T> {
> +        if self.len() >= self.capacity() {

len() > capacity() should be impossible by the (implied until
https://lore.kernel.org/rust-for-linux/20250318-vec-set-len-v2-1-293d55f82d18@gmail.com/)
type invariant.

> +            return Err(v);
> +        }
>
>          // SAFETY:
>          // - `self.len` is smaller than `self.capacity` and hence, the resulting pointer is
>
> --
> 2.49.0.rc1.451.g8f38331e32-goog
>
>